Recently, laminate batteries in which a battery element is sealed by external cladding (laminate sheets) have drawn attention as a form of a secondary battery such as a lithium battery. Japanese Patent Application Publication No. 2004-052100 A discloses a method for manufacturing the external cladding on which a projecting portion for housing the battery element is formed by using austenitic stainless steel foil as a material, and by implementing drawing on the stainless steel foil at room temperature. By employing the external cladding made of the stainless steel foil in this manner, a strong and lightweight laminate battery can be constructed.
This type of laminate battery is applied to electric automobiles or the like, and therefore requires a large capacity in order to extend the range of electric automobiles for instance. To increase the capacity of the laminate battery, a space that can accommodate a larger battery element must be secured. With the configuration described above, however, drawing is implemented on the stainless steel foil at room temperature, and therefore molding defects such as cracks occur when an attempt is made to form a deep projecting portion.
Japanese Patent Application Publication No. 2009-113058 A discloses a configuration for realizing deep drawing, when drawing is implemented on austenitic stainless steel sheet, by performing warm working in which a region of the stainless steel foil that contacts a punch is cooled, a region on an exterior thereof is heated, and the punch is pressed into the stainless steel foil while supplying lubricating oil.
The present applicant considered the application of drawing such as that described in Japanese Patent Application Publication No. 2009-113058 A to manufacture external claddings for laminate batteries such as that described in Japanese Patent Application Publication No. 2004-052100 A, but found that the following problems arise. The method described in Japanese Patent Application Publication No. 2009-113058 A requires the performance of washing for degreasing after the drawing because the drawing is performed while supplying lubricating oil. When a laminate battery is constructed using external claddings manufactured by this drawing process, however, the lubricating oil as well as dust and the like adhered to the lubricating oil may infiltrate interior of the battery, causing the battery to malfunction. If the lubricating oil is simply not used, on the other hand, the material can no longer be moved smoothly, and therefore deep drawing cannot be realized.